1. Field of the Invention
The present invention relates in general to a solid state image pickup device, and more particularly to a solid state image pickup device having an analog-to-digital converter (hereinafter referred to as “an A/D converter” for short when applicable).
2. Related Background Art
In recent years, the importance of a solid state image pickup device used in digital imaging apparatuses has more and more increased along with the spread of a digital still camera, a digital video camera, an image scanner, and a mobile phone with a digital camera.
In many cases, in each of these digital imaging apparatuses, analog signals obtained from the solid state image pickup device are converted into digital signals by an A/D converter which is separately installed in order to execute image processing or the like for the resultant digital signals.
However, in the recent progress in the miniaturization and the promotion of low cost of these digital imaging apparatuses, it has been required to provide the A/D converter in the solid state image pickup device.
Now, heretofore, in the solid state image pickup device with an A/D converter, as described in JP 09-009017 A as Japanese Patent Application Laid-open for example, it is the main processing to output data after the digital conversion in parallel with one another. In this case, output terminals are required corresponding to the number of bits of the data. For example, in case of a solid state image pickup device having an A/D converter for 8 bits, eight output terminals are required, and in case of a solid state image pickup device having an A/D converter for 12 bits, twelve output terminals are required.
For this reason, in the solid state image pickup device having a relatively small image pickup area, a ratio of an area occupied by the output terminals becomes relatively large as compared with the overall chip size. This becomes a factor of increasing a chip cost. In addition, in a package in which a solid state image pickup device is sealed, an increase in the number of output terminals results in an increase in the package size. This is a factor of increasing a package cost.
The above-mentioned tendency becomes more remarkable when an image sensor and an A/D converter, and moreover a timing generator are formed on the same semiconductor substrate.
Furthermore, in recent years, the radiation noises generated by an electric apparatus are questioned with respect to an influence exerted on the human body or other electric apparatuses. These radiation noises are generated by a current which is caused to instantaneously flow through a wiring and the like when a voltage changes from a high level to a low level or from a low level to a high level. In case of the parallel output style, there is a possibility that the data of the bits changes at the same timing. Hence, the peak values of the radiation noises further increase as the number of bits of the A/D converter further increases, i.e., the number of output terminals further increases.
For this reason, an attempt has been made such that the data after the digital conversion is outputted in serial with one another in order to reduce the number of output terminals for digital data.
FIG. 10 is a diagram explaining the processing for serial data outputted by a conventional solid state image pickup device with a built-in A/D converter of a serial data output type. In the figure, reference numeral 510 designates a solid state image pickup device with a built-in A/D converter of a serial data output type, reference numeral 500 designates a start pulse for regulating start of an operation of the solid state image pickup device 510 with a built-in A/D converter, reference numeral 501 designates serial output data of 8 bits into which analog signals outputted from respective pixels are digitally converted, reference numeral 503 designates a clock pulse, reference numeral 521 designates a time period from a time point of input of the start pulse 500 to a time point of output of the effective data, reference numeral 523 designates a time period required to output one data, and reference numeral 505 designates a microprocessor (MPU) for image processing. As can be seen from the figure, in order that the microprocessor 505 for image processing may discriminate a break between the serial data outputted from the solid state image pickup device 510, first of all, the start pulse 500 is detected, and the number of clocks for the predetermined time period 521 is then counted to thereby recognize the head bit of the effective data. Thereafter, the clock pulses for eight periods within the time period 523 required to output one data are counted to thereby recognize the head bit of next one data. Such processing is repeatedly executed to thereby discriminate a break between the serial data two by two. For discriminating the break between the serial data in such a manner, the number of clocks needs to be always counted.
As described above, in case of the serial output style, the digital data is continuously outputted through one terminal. Thus, unlike the case where the data is outputted in parallel with one another, as the processing for discriminating the break between the data two by two, it becomes necessary to always count the number of clock pulses. For this reason, the load imposed on the microprocessor for image processing in a latter stage becomes heavier than that in the case where the parallel output data is processed.